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2004 Microchip Technology Inc. DS00914A-page 1
AN914
INTRODUCTION
Dynamic memory allocation is a nice functionality that
is provided with virtually all PC-based compilers. How-
ever, not all microcontroller compilers have such capa-
bility, most likely due to the lack of a sophisticated
operating system with memory management. Although
most applications are static in nature, there are cases
where a need for dynamic allocation of memory
resources exists. Examples include any number of net-
work protocols that have a dynamically specified
nature. This application note presents a simple and effi-
cient method for dynamic memory allocation without
the need of an operating system.
THE MODEL
The model is based on a simple form of a linked list. A
block of memory referred to as the dynamic heap is
split into segments. Each segment has a single-byte
header that references the next segment in the list via
an offset, as well as indicating whether the segment is
allocated. Allocation is specified by a single bit.
Figure 1 shows an example. Consequently, the
reference implicitly identifies the length of the segment.
The heap is terminated with a special header that
references itself, referred to as the “tail”.
Why use single-byte headers? The segment headers
are specifically designed to be a single byte wide to
achieve excellent execution performance, reduce code
size and minimize loss of memory space to segment
control information. Essentially, one byte references are
easier and faster to manipulate than multi-byte relative
or absolute references. Plus, they do not consume as
much space. However, some fundamental limits are
imposed by this methodology. The maximum segment
size is 126 bytes, or the size of the heap, whichever is
smaller. The smallest segment size is one byte, resulting
in a maximum number of segments of one-half of the
number of bytes in the heap minus one. For example, in
a 512-byte heap, one could expect to dynamically
allocate as many as 255 single-byte segments.
FIGURE 1: SIMPLE HEAP EXAMPLE
Although this model will allow dynamic allocation down
to a single byte, doing so sacrifices performance and
memory. With more segments within the heap, more
time is required to attempt to allocate memory. In addi-
tion, every segment requires a header byte; therefore,
a large number of smaller segments require more
memory than a small number of large segments. In the
255-segment example mentioned previously, 50% of
the heap is lost to segment header information.
There is also one other potential problem, especially
with smaller segments: memory fragmentation. Frag-
mentation could ultimately doom an application by
reducing the largest allocatable block of memory. Thus,
dynamic allocation should be restricted to larger blocks
to maintain efficiency and effective use of the heap.
Applications that are likely to encounter fragmentation
issues should provide a method to handle allocation
failures. The implementation depends on the complex-
ity of the application. For some applications, a system
Reset may be sufficient. For applications with more
advanced memory requirements, it may be necessary
to provide allocation management functions. An exam-
ple might be to force non-critical tasks to give up their
memory allocations as needed, then re-allocate
memory to them as required.
Author: Ross M. Fosler
Microchip Technology Incorporated Allocation Bit
Memory Heap
Segment
Length/Reference
Segment 1
Segment x
Tail
Dynamic Memory Allocation for the MPLAB® C18 C Compiler
Produktspecifikationer
| Varumärke: | Microchip |
| Kategori: | Inte kategoriserad |
| Modell: | PIC18F2520 |
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